Project summary The epithelial sodium channel (ENaC) fine-tunes Na+ and water balance and is the primary regulator of blood pressure and volume through Na+ reabsorption in the distal nephron. Thus, ENaC is a critical therapeutic target for kidney diseases and blood pressure control. Here we propose to develop a molecular model for ENaC function by determining high-resolution structures of ENaC in different functional states. Molecular studies of ENaC have proven difficult largely because ENaCs are labile, multimeric integral membrane proteins that are activated by proteolysis of inhibitory domains located in the extracellular domain; how these peptidyl tracts exert their effects remains unresolved. Our goal is to develop methods for expression and purification of ENaC in the uncleaved and cleaved conformations. We will apply these methods to elucidate structure-based mechanisms underpinning ENaC gating, ion permeation, and allosteric modulation by combining single-particle cryo-electron microscopy and electrophysiology, together with judiciously chosen biochemical and biophysical assays. The structures will offer key insight into the molecular basis of proteolytic activation of ENaC providing positions of the inhibitory domains and identifying critical regions that mediate ENaC acitivity yielding the first accurate blueprints for rational, structure-based therapeutic strategies.